1. Field of the Invention
The present invention is useful in color signal processing. It consists of a color signal separating circuit using a color signal separating algorithm.
2. Description of the Related Art
A digital signal processing camera using a CCD area image sensor (hereinafter referred to as a CCD) converts an image signal composed of light into an electrical signal representing the original image. It then performs predetermined signal processing on the electrical signal. Next, it saves the processed signal in a storage medium such as an analog video tape recorder or digital video tape recorder so that the electrical signal can be reproduced when necessary. There are two kinds of CCDs: one is for black-and-white signal processing, and the other is for color signal processing. Based on the CCD they contain, cameras are divided into groups of black-and-white or color, and each type employs different basic signal processing methods. Black-and-white signal processing is simple since photoelectric conversion requires only one characteristic, the brightness of an image. On the other hand, color signal processing requires luminance signals representing the brightness of an image as well as color signals to keep track of colors.
The present invention is confined to color signal processing. To convert a two-dimensional light image signal into a color signal, there are two methods. First, one can install three primary color filters (red, blue, and green) into a CCD and convert the light image into an electrical signal. Second, one can install a prism, obtain three primary red, blue, and green colors, and then convert each resulting light image into electrical signals. In the latter, color separation is excellent, although the size of a camera is increased due to the need for a prism and three black-and-white CCDs. On the other hand, the former has disadvantages in that it is difficult to build due to the increase of CCD pixels. It also suffers from poor color reproduction quality due to uniformless red, blue, and green filter characteristics. However, it has the advantage of creating small cameras. To get the best of both methods, single CCD cameras have been widely used recently. They process a image signal with a single CCD, then use a complementary color filter exhibiting excellent filter characteristics.
A conventional color signal separating technology using a complementary color filter will be described referring to FIGS. 1 and 2.
FIG. 1 illustrates a CCD complementary color filter array.
Referring to FIG. 1, the complementary color filter array includes magenta (MG) composed of red (R) and blue (B) signal components; yellow (YE) composed of red and green (G) color signal components; cyan (CY) composed of blue and green signal components; and a green signal G. Signals read from the complementary color filter array of FIG. 1 are divided into even fields and odd fields.
FIG. 2 is a timing diagram of output signals of a CCD having the complementary color filter array shown in FIG. 1.
Referring to FIG. 2, S2(=MG+YE) and S1(=G+CY) are sequentially output from an odd line of an odd field, while S2(=G+YE) and S1(=MG+CY) are sequentially output from an even line of the odd field. In addition, S2(=G+YE) and S1(=MG+CY) are sequentially output from an odd line of an even field, and S2(MG+YE) and S1(G+CY) are sequentially output from an even line of the even field. To reiterate, in the even field a signal component stored in each pixel is output as synthesized MG+YE and G+CY in the even line, and as synthesized G+YE and MG+CY in the odd line, by the complementary color filter defined by a field interlace method. Symmetrically, in the odd field the signal is output as synthesized G+YE and MG+CY in the even line, and as synthesized MG+YE and G+CY in the odd line. This is a formal format regulated in a CCD structure, and defines an interlace signal processing method. The signals generated above are the fundamental inputs to the color signal processing algorithm of a color camera.
In conventional color signal separating technology, the above four signals are separated into pseudo color signals and luminance signals by the following algorithm.                               Even          ⁢                      xe2x80x83                    ⁢          field                ,                                            odd              ⁢                              xe2x80x83                            ⁢              line              ⁢                              :                            ⁢                              xe2x80x83                            ⁢              S2                        -            S1                    =                      xe2x80x83                    ⁢                                    (                              G                +                YE                            )                        -                          (                              MG                +                CY                            )                                                              =                  xe2x80x83                ⁢                  G          +          R          +          G          -          R          -          B          -          G          -          B                                        =                  xe2x80x83                ⁢                  G          -                      2            ⁢            B                                                  =                  xe2x80x83                ⁢                  -                      (                                          2                ⁢                B                            -              G                        )                                                  =                  xe2x80x83                ⁢        CCB                                          Even          ⁢                      xe2x80x83                    ⁢          field                ,                                            even              ⁢                              xe2x80x83                            ⁢              line              ⁢                              :                            ⁢                              xe2x80x83                            ⁢              S2                        -            S1                    =                      xe2x80x83                    ⁢                                    (                              MG                +                YE                            )                        -                          (                              G                +                CY                            )                                                              =                  xe2x80x83                ⁢                  R          +          B          +          R          +          G          -          B          -          G          -          G                                        =                  xe2x80x83                ⁢                              2            ⁢            R                    -          G                                        =                  xe2x80x83                ⁢        CCR                                                      S2            +            S1                    =                      xe2x80x83                    ⁢                                    [                                                (                                      G                    +                    YE                                    )                                +                                  (                                      MG                    +                    CY                                    )                                            ]                        ⁢                          xe2x80x83                        ⁢            or                          ⁢                  xe2x80x83                                                  xe2x80x83                ⁢                  [                                    (                              MG                +                YE                            )                        +                          (                              G                +                CY                            )                                ]                                                  =                      xe2x80x83                    ⁢                                    (                              R                +                G                +                B                +                G                +                G                +                R                +                G                            )                        ⁢                          xe2x80x83                        ⁢            or                          ⁢                  xe2x80x83                                                  xe2x80x83                ⁢                  (                      R            +            G            +            B            +            G            +            G            +            R            +            G                    )                                        =                  xe2x80x83                ⁢                              2            ⁢            R                    +                      3            ⁢            G                    +                      2            ⁢            B                                                  =                  xe2x80x83                ⁢        YL            
Though not shown here, CCR and CCB outputs are opposite in an odd field. Necessary red, blue, and green signal components are separated using pseudo color signals CCB and CCR and a pseudo luminance signal YL obtained by the above algorithm. An algorithm for this color separation is given as follows.                     Red        =                  xe2x80x83                ⁢                  CCR          +                      0.12            ⁢            YL                                                  =                  xe2x80x83                ⁢                              2            ⁢            R                    -          G          +                      0.12            ⁢                          (                                                2                  ⁢                  R                                +                                  3                  ⁢                  G                                +                                  2                  ⁢                  B                                            )                                                              =                  xe2x80x83                ⁢                              2            ⁢            R                    -          G          +                      0.24            ⁢            R                    +                      0.36            ⁢            G                    +                      0.24            ⁢            B                                                  Green        =                  xe2x80x83                ⁢                  YL          -                      CR            ⁡                          (                                                CR                  =                                      CCR                    -                    CCB                                                  ,                here                            )                                                              =                  xe2x80x83                ⁢                              2            ⁢            R                    +                      3            ⁢            G                    +                      2            ⁢            B                    -                      [                                          (                                                      2                    ⁢                    R                                    -                  G                                )                            +                              (                                                      2                    ⁢                    B                                    -                  G                                )                                      ]                                                  =                  xe2x80x83                ⁢                  5          ⁢          G                                        Blue        =                  xe2x80x83                ⁢                              -            CCB                    +                      0.2            ⁢            G                                                  =                  xe2x80x83                ⁢                              2            ⁢            B                    -          G          +                                    0.2              H                        ⁢            5            ⁢            G                                                  =                  xe2x80x83                ⁢                  2          ⁢          B                    
A second color separation algorithm is also commonly used for camera signal processing. This algorithm, though using a complementary color filter and the same basic signals output from a CCD, employs a different signal processing method and thus further improves the color quality the electrical image signal relative to the previous algorithm.
MG=R+B
YE=R+G
CY=G+B
G=G
C1=MG+YE=2R+B+G
C2=G+CY=2G+B
C3=MG+CY=2B+R+G
C4=G+YE=2G+R                     Red        =                  xe2x80x83                ⁢                              C1            -                                          SR                H                            ⁢              C2                                =                                    2              ⁢              R                        +            B            +            G            -                          0.5              ⁢                              (                                                      2                    ⁢                    G                                    +                  B                                )                                                                            =                  xe2x80x83                ⁢                              2            ⁢            R                    +                      0.5            ⁢            B                                                  Blue        =                  xe2x80x83                ⁢                              C3            -                                          SB                H                            ⁢              C4                                =                                    2              ⁢              B                        +            R            +            G            -                          0.5              ⁢                              (                                                      2                    ⁢                    G                                    +                  R                                )                                                                            =                  xe2x80x83                ⁢                              2            ⁢            B                    +                      0.5            ⁢            R                                                  Green        =                  xe2x80x83                ⁢                              (                          C2              +              C4                        )                    -                      SG            ⁡                          (                              C1                +                C3                            )                                                              =                  xe2x80x83                ⁢                              2            ⁢            G                    +          B          +                      2            ⁢            G                    +          R          -                      0.33            ⁢                          (                                                2                  ⁢                  R                                +                B                +                G                +                                  2                  ⁢                  B                                +                G                            )                                                              =                  xe2x80x83                ⁢                              4            ⁢            G                    +          B          +          R          -                      0.33            ⁢                          (                                                3                  ⁢                  R                                +                                  3                  ⁢                  B                                +                                  2                  ⁢                  G                                            )                                                              =                  xe2x80x83                ⁢                              4            ⁢            G                    -                      0.66            ⁢            G                    +                      0.01            ⁢            B                    +                      0.01            ⁢            R                                                  =                  xe2x80x83                ⁢                              3.34            ⁢                          xe2x80x83                        ⁢            G                    +                      0.01            ⁢                          xe2x80x83                        ⁢            B                    +                      0.01            ⁢            R                              
The above algorithm is advantageous in that color quality is improved, when compared to the first algorithm, by intersectionally operating on adjacent signal components. Unfortunately, in the above two algorithms other components are included in the basic signals red, blue, and green. These cause the phase of a color to vary during formation of three important signals generated in subsequent stages: color difference, white balance, and hue control. The net result of this is unintended colors. Moreover, in the above conventional algorithms, differences made between luminance signals by the filter causes the incorrect reproduction of red and blue signals. Finally, due to the prior mentioned luminance signal difference, the level of a green color signal varies as the basis for generating red, green, and blue color signals at every line. This decreases the color quality of green.
Digital cameras use CCD image sensors to transform images from light waves into electrical signals. There are two types of CCD sensors, those designed for black-and-white imaging and those designed for color. Black-and-white are very simple, as they must only determine the brightness of the elements of an image, while color detection is much more complicated as the CCD must detect both the brightness (luminance) of the elements as well as the colors.
In today""s cameras, a single CCD using a complementary color filter does this processing. The filter works via an array of four colors, magenta, yellow, cyan, and green. The array is broken up into even and odd fields, each of which has an odd and even line. Values of each pixel are clocked out sequentially, each pixel being represented by a synthesized MG+YE and G+CY in the even line of the even field, and a synthesized G+YE and MG+CY in the odd line of the even field. The same process (with swapped color codes according to FIG. 1) happens in the odd fields. These values are then processed by a color signal separating circuit, and the background of the invention describes two methods for determining the luminance and color from these sampled pixel values in the prior art.
This invention is the derivation and implementation of a new method for performing this color separation. This method has several advantages over the prior art. These include a lack of other RGB signal components in each basic color; a reduction in the difference between luminance signal; and improved color quality of adjacent (odd and even) lines due to using a stable green signal to obtain the red and blue signals. The circuit designed to implement this consists of a CCD Output Synchronizing Block, a Color Signal Generating Block, and a Pre-White Balance Controlling Block.